1. Field of the Invention
This invention relates to a method for driving an electrically luminous material, such as so-called electroluminescence (referred to hereinafter as EL) display panel of a matrix type, particularly in which a scan electrode of a matrix type display panel is driven by a pulse which is composed of a pedestal pulse and a scan pulse. More particularly, it relates to an improvement for achieving uniformity of brightness all over the panel.
2. Description of the Related Art
As is well known, in a matrix type EL display panel, an EL cell located at an intersection of a scan electrode and a data electrode is selectively lit by application of a pulse voltage on the scan electrode, as well as by simultaneous application of a data pulse voltage, on a data electrode, having a polarity opposite to that of the scan pulse. The applied pulse voltage between the scan and the data electrodes is the sum of the respective absolute values of the scan pulse voltage and data pulse voltage, and is called a cell voltage. Polarity of the cell voltage is generally altered with every frame cycle in order to attain brighter light output as well as normal operation of the EL cell. Furthermore, in order to lower the voltage rating of the scan driver, the scan pulse is composed of a pedestal pulse whose duration is approximately 15.0 ms, for example, for the frame cycle time 16.7 ms of 60 frames per second, and an additional scan pulse of 25 to 30 .mu.s, for example, for 400 scan electrodes. The pedestal pulse has been popularly employed for driving a PDP (plasma display panel), and it is also disclosed in the International Publication Number of PCT: WO 83/03021 by HARJU, Terho, Teuvo. FIG. 1(a) illustrates a waveform of a data electrode, where V.sub.dp indicates a data pulse having a positive voltage Vd, and having a pulse width basically the same as that of the scan pulse, thus, a single cell at the center of a data electrode is lit. FIG. 1(b) illustrates a waveform of a scan electrode, where V.sub.pp indicates a pedestal pulse having a negative voltage -V.sub.p and the pulse width T.sub.p. V.sub.sp indicates a scan pulse having a negative voltage -Vs superposed on the negative pedestal pulse V.sub.pp. Thus, when a scan electrode is selected, the total scan pulse voltage Vp+Vs, a so-called half-selective voltage, is applied thereto. The level of the individual pulse Vd or Vp+Vs is chosen not adequate to light the cell by itself. FIG. 1(c) illustrates a wave form of a cell voltage of the cell to which the above-mentioned data pulse and scan pulse are applied, measured with reference to the scan electrode. The peak level Va, which is the sum of the absolute values of Vd, Vp and Vs, is chosen high enough to light the cell, such as 215 V, thus this peaked pulse is called a write pulse. With this constitution of these driving pulses, the scan driver which is composed of an integrated circuit (IC) has to only switch a low voltage Vs, such as 25 V, which, in other words, is the difference between the half-selective pulse voltage 190 V and the pedestal pulse level 165 V, therefore is much less than the total scan voltage, 190 V. Timing charts of these pulses are shown in FIG. 2, where n scan electrodes are provided. FIG. 2(a) shows the waveform of the i-th data electrode. FIG. 2(b) through (d) show respective waveforms of the scan electrodes S.sub.1 through S.sub.n. FIG. 2(e) through (g) show respective waveforms of the cell voltage at intersections of the i-th data electrode and respective scan electrodes S.sub.1 through S.sub.n. Generally the data electrodes D.sub.1 through D.sub.m are driven in parallel by the data driver 6-l through 6-m. Therefore, the frame cycle time T.sub.f is the time required to scan all the scan electrodes (as many as n) and then to return to the first scan electrode for the next frame cycle. During the next (i.e. k+1 th) frame cycle, the pedestal pulse is generally reversed and the level is +190 V, whose absolute value is different from that of the previous frame cycle, because the data electrode is biased at a high level (+25 V) to deliver a data pulse of 0 V to the data electrodes. Thus, a write pulse of the same height as that of the previous cycle is produced.
A typical scan pulse generator/driver for delivering the pulses is shown in FIG. 3, as quoted in the above-mentioned patent application. However, in this method employing the pedestal pulse, there is a problem in that the brightness of a particular lighted cell varies depending on the number of the lighted cells connected to the same data electrode through a frame cycle. Description regarding this problem hereinafter is made for a still picture, where the data pulses are same for every frame cycle, in order to simplify the explanation. FIG. 1' illustrate an extreme case, where all the cells on a data electrode are lit, in comparison with FIG. 1 where a single cell on a data electrode is lit. As observed in FIG. 1'(c), the pedestal pulse level becomes virtually Vp+Vd, because data pulses for lighting all the cells are continuously superposed on the pedestal level. Brightness characteristics of these two cases depending on the virtual pedestal level are shown in FIG. 4, where the level of the write pulse is variable. The curve "b" is of the case having a 165 V pedestal pulse to simulate the single lighted cell of FIG. 1, and curve "c" is of the case having a 190 V pedestal pulse to simulate the all-lighted cells of FIG. 1'. The brightness of the curve "c" is obviously lower than that of the curve "b". Brightness characteristics of the case where the number of the lit cells is between "a single cell" and "all cells" must come to between the curve "b" and "c". In order to simulate this state, curve "d" of FIG. 6 is obtained by measuring a sample EL panel. In FIG. 6, the pedestal voltage is variable while the write pulse and the data pulse are kept constant, respectively 240 V and 25 V. Accordingly "the pedestal pulse voltage+the scan pulse voltage" is kept constant. As observed with the curve "d" the brightness decreases as the virtual pedestal voltage is increased over 150 V. This means that the brightness decreases as the number of the lit cells on a data line increases. As is well known, when the EL material produces a light by a write pulse, electrical charges in the EL material, as a dielectric material, are displaced by the applied electric field causing a charge polarization.
The mechanism of this phenomena is hereinafter explained. Brightness of the produced light of a cell depends on the amount of the produced polarization charge therein. The relation between the applied pulse and the produced polarization charges in the cell was investigated and is shown in FIG. 7. The solid lines show the case where a single cell on a data electrode is lit, and the dotted lines show the case where all the cells on a data electrode is lit. The timing when the pedestal pulse is applied to the scan electrode is indicated by "tp", and the timing when the write pulse is applied to the electrodes is indicated by "tw". The polarization charge remaining before tp is the residual charge of the previous frame cycle, during which the polarity of the cell voltage was reversed. The increment of the charge curve "f" of the low pedestal level ("a single lit cell") at tw is larger than that at tp. The word "increment" used above as well as hereinafter means the difference of the charge between before and after the application of a pulse voltage, and the word "difference" includes not only the difference in a particular polarity of the charge but also the charge difference from the plus charge to the negative charge and vice versa. However, the increment of the curve "g" of the high pedestal level ("all the lit cells") at tw is smaller than that at tp. Furthermore, over the all increment Qb (0.38 .mu. coulomb/cm.sup.2) of the curve " g", is smaller than the over all increment Qa (0.48 .mu. coulomb/cm.sup.2) of the curve "f". This data means that the difference of the virtual pedestal level gives effect on the charge increment at tp as well as over all increment, accordingly causing the deterioration of the brightness characteristics.